There is such a conventionally known producing process disclosed in Japanese Patent Application Laid-open No.6-328509. In this case, the maximum value of an injection speed V.sub.2 of the core forming material at the second step is set to be lower than the maximum value of an injection speed V.sub.1 of the outer layer forming material at the first step.
In a producing process of such type, while the outer layer forming material is flowing within the cavity, the core forming material is allowed to flow into the outer layer forming material. However, after a front portion of the outer layer forming material in a flowing direction reaches a terminal end of the cavity to stop its flowing, it is difficult to allow the core forming material to flow into the front portion of the outer layer forming material in the flowing direction.
If the injection speed is set as in the prior art under such a circumstance, the following problem is liable to arise: The front portion of the core forming material in the flowing direction has not yet flown into the front portion of the outer layer forming material in the flowing direction at a time point when the front portion of the outer layer forming material in the flowing direction reaches near the terminal end of the cavity. Due to this, it is impossible to allow the core forming material to flow into the front portion of the outer layer forming material in the flowing direction and hence, it is impossible to produce a synthetic resin-molded article having a sandwich structure over the whole thereof.